The invention relates to a process producing batches of a mouldable, fibre-reinforced thermoplastic material matched to the mouldings to be manufactured, accompanied by heating.
The reinforcing fibres are in particular constituted by cut glass fibres, but it is also possible to use other mineral, inorganic or miscellaneous synthetic fibres, such as carbon, aramide or polyester fibres. When reference is made hereinafter to glass fibers, the latter can be replaced by the aformentioned fibres. Cut glass fibers are used in large quantities for the production of fibreglass-reinforced plastic parts. The glass fibres are bound by a binder, which has as the main component or matrix a thermoplastic material, such as polypropylene and in addition can contain carbonblack, was and other additives. In one form, the production of a semifinished product in a press to give a moulding takes place by strewing the fibres to form a fleece and impregnating the same with liquid synthetic resin. In another form, the fibers are processed in the liquid phase to a suspension or slurry, which is processed by stirring or mixing movements to give a tangled fibrous material. Here again, use is made of liquid synthetic resins or pulverulent binders in conjunction with an aqueous suspension. In the latter case, the suspension is dried after forming a tangled fibre fleece. In both case the thickness and width of the fleece must be adapted to the mouldings to be moulded, the former being necessary and the latter in order to avoid excessive waste. The fleece must subsequently be cut into individual plates adapting to the mouldings to be produced.
It has also been proposed (German Offenlegungsschrift 36 04 888 and the equivalent British Patent 21 90 916) to add liquid wetting agents with a maximum proportion of 20% by weight, so as to obtain a moist, but still free-flowing material, which as an intermediate product is packed in plastic bags and transferred to the further processing enterprise or can also be processed in a heated belt press to preshaped plate material.
In particular, the first-mentioned processes are very complicated and involve high costs. In all the aforementioned processes the preparation and compounding of the semifinished product is necessary, which makes the process inflexible and prevents or makes much more difficult adaptations to other mouldings.
It has also already been proposed to whirl glassfibre bundles together with the thermoplastic material-containing binders in a whirling or turbulence chamber to a cottonwool or wadding-like felt and subsequently either to pack under a vacuum, preferably in plastic bags made from a material compatible with the thermoplastic material of the binder and in particular the same material, or to make same directly available for further processing (German patent application P 37 04 035.9) and the equivalent U.S. Pat. No. 4,886,701.
The aim underlying the present invention essentially resider in providing a process which does not presuppose the preparation and compounding of the starting material as a function of the mouldings to be produced and which is in particular suitable for further processing of the starting material produced according to the aforementioned process, but also for processing unmatched intermediate products.
According to the invention this problem is solved by a process which is characterized in that an initial product of fibers and thermoplastic material-containing binder is heated as a strand and a matched batch is separated from the strand and supplied for batched further processing. The process is characterized in that there is a melting device with a melting channel, through which the material to be processed is forced as a continuous strand, as well as a dosing station.
The inventive process simplifies the production sequence, as well as permitting a high level of automation during moulding production. No preparation and compounding of the starting product are required so that costs can be kept low. The inventive process may be carried out in a closed system, so it is possible to prevent decomposition of the thermoplastic matrix by overheating so that fire risks are reduced and it is less prejudicial to the environment. The inventive process permits considerable variation possibilities regarding the use of higher-quality thermoplastics (e.g. PA and PTP), the use of different reinforcing fibres and a change to the batch volume for the moulding at the press, so that the inventive apparatus by means of the process for providing the changes can be used for several following, presses, which also differ as regards the charge quantity for different mouldings.
In particular, the starting material in free-flowing form can be supplied in a standard pack size, such as a vacuum bag, or a dimensionally stable, precompressed semifinished product and need not, as is necessary in the case of glass mat production, be adapted in accordance with special use weights, etc. to the mouldings. The dosing carried out at the end of the inventive process or on leaving the inventive apparatus can then be carried out precisely on the basis of the moulding to be produced and can be changed between individual batches, so that several presses, which produce different mouldings and also with different weights can directly follow one another and can be alternately supplied by the dosing station.
According to a preferred development of the invention, the initial product is precompressed and directly supplied to the continuous strand or the melting device has upstream thereof a compressing station with a common channel for the material to be processed. According to a further development the compressing station has a cooled compressing channel, to which is connected the melting channel of the melting device and at the end of the compressing channel remote from the transition point of compressing channel and melting channel is provided with a movably positioned compressing and feed plunger. In particular the melting channel of the melting device is surrounded by heating elements over its entire length. The dosing piston of the dosing device can simultaneously be the feed piston for passing the softened material through the melting device. According to a preferred development, vacuum is applied when compressing the starting material, so as to avoid air inclusion and the like. For this purpose and in particular in the vicinity of the compressing station, vacuum lines can be provided for applying the vacuum. According to a preferred development the melting channel is provided over its entire length with heating elements. The material is heated in the melting channel to a temperature which is higher than the melting point of the thermoplastic material and is in particular 40.degree. to 100.degree. C. over the melting point. The inventive process permits a very high throughput of material to be softened and dosed, so that it is readily possible to achieve a throughput of 2 Kg or more per minute. If the heating channel has a considerable length, it is preferably laid in loops, so as to shorten the overall length of the apparatus.
In order to keep the actual heating channel surrounded by heating elements, e.g. electric heating coils, in which the material to be processed is kept pasty-liquid, as short as possible, according to a preferred development, the compressed initial product is heated over its entire volume by hot gas or at least the fibrous core region of the semifinished product is heated by hot gas. If free-flowing or pourable product is supplied, it is possible to have between the cooling station which prevents the material from sticking to the compressing plunger and the compressing chamber surrounded by the heating elements, a hot air station constructed in such a way that a corresponding channel area, through which the material is pressed is surrounded by a metal filter, which forms the wall of the area and through whose openings radially hot gas and in particular inert hot gas, such as nitrogen or helium, if the polymer is not attacked also hot air is forced through, which heats the material in said area over its entire cross-section and therefore volume to above the melting point of the polymer.
According to another development the precompressed, dimensionally stable semifinished product is supplied. In particular semifinished product with a rigid outer covering and an inner felt-like core of fibreglass-polymer-granular mixture can be supplied. The polymer can in particular be in dust or granule form. Such a dimensionally stable semifinished product can be produced by brief, very high heating of a covering area of felt-like starting material in the compressed state.
So that also in this case the melting chamber can be made relatively short, a hot air station is connected upstream of the melting channel. In this case, the hot air station is positioned in front of an inlet to the compressing and melting station. It can in particular have a temperature control chamber, which can preferably be bounded by two displaceable pistons, but also by other suitable devices, through which the dimesionally stable semifinished product is moved from a supply opening to a discharge opening to the compressing and melting station. Openings are provided in the two pistons through which hot air is blown axially into the area between the two pistons and through the felt-like core area of the semifinished product in order to heat the latter here to a temperature just below the melting point. At such a temperature the semifinished product is conveyed on to complete melting and dosing, as described hereinbefore. This also makes it possible to significantly shorten the melting channel surrounded by the heating elements.
Further advantages and features of the invention can be gathered from the claims and following decription of an embodiment of the inventive process for providing a matched quantity of a mouldable fibre-reinforced thermoplastic material with reference to the attached drawings: